Head-mounted and heads-up display products have been created and marketed for decades. Early versions used image sources such as cathode ray tubes, which evolved to include liquid crystal displays (“LCD”) and organic light emitting diode displays (“OLED”). The eyepiece optics evolved from single and multi-element glass spherical lenses to low-cost, lightweight aspheric plastic lenses.
With the advent of smartphone displays having high pixel densities in the range of 300 to 600 pixels per inch over diagonal screen sizes of 4 inches or larger, it is possible to display reasonably high resolution graphic and video content in a split-screen, landscape mode (side-by-side) format such that a common image or half of a 3D stereo pair is presented to each of the viewer's eyes. With appropriate optics and viewing geometries, video content meeting the 720P video standards or better can be achieved with a premium smartphone and appropriate optics. Visual fields of view in excess of 70 degrees diagonal can be achieved, which is equivalent to viewing a 50 inch TV monitor from a distance of 3 feet or less. So the viewing experience can be fairly immersive. To meet the market demands of such products, the lenses have to be compact, lightweight, inexpensive and high-performance. Such demands are not simultaneously achieved by conventional optical designs.
There are many viewer products presently addressing this market, such as Google Cardboard, a virtual reality platform developed by Google for use with a head mount for a smartphone, which incorporates a fold-out cardboard viewer; Oculus Rift, a virtual reality headset developed and manufactured by Oculus VR; Samsung Gear VR, a mobile virtual reality headset developed by Samsung Electronics; and many others. The eyepiece optics in such products range in visual quality from very poor to very good. In general, they strive for a wide field of view for an immersive visual experience. Some products strive for low-distortion optics design while others use compensating software predistortion to compensate for the lens deficiencies. Some products use thinner, lightweight, low-cost Fresnel lenses. However, most ignore chromatic aberation, resulting in color separation that increases with field of view. This is detrimental to resolution of fine detail, especially small text.
In general, use of diffractive optics in imaging optics to achieve chromatic correction, including combining such diffractive optic surfaces with refractive optics, can be found in the prior art. Characteristics of some such optical systems are summarized in Missig and Morris, “Diffractive Optics Applied to Eyepiece Design” (Applied Optics, Vol. 34, No. 14, 10 May 1995), with numerous historical references. Such techniques have previously been used in the development of numerous products for Kopin Corporation of Westborough Mass.
For example, Chen U.S. Pat. No. 5,151,823 discloses a three element binocular eyepiece having a diffractive surface etched onto a lens surface facing the object needing magnification and a four element lens having a diffractive surface etched onto the third element from the entrance pupil. Chen also discloses a Fresnel lens element having a multi-level diffractive optic structure disposed thereon. As another example, Johnson U.S. Pat. No. 5,161,057 addresses a dispersion-compensated lens for solar concentration wherein diffractive surface structures are superimposed on a Fresnel groove structure for chromatic correction. Such combined Fresnel-diffractive surfaces are not well suited to visual imaging applications as diffractive features and Fresnel features can chromatically interfere with one another.
Also, the combination Fresnel and described low-diffractive Fresnel is discussed and claimed in Hunter U.S. Pat. No. 5,347,400 for a virtual reality helmet. However, this patent does not disclose or claim the combination for chromatic correction; rather it is directed to minimizing Moire' fringing interference between surfaces and to optimizing off-axis visual quality with a wide field of view.
A diffractive-Fresnel combination is also disclosed in Hebert U.S. Pat. No. 5,926,318 as applied to a biocular micro display head-mounted system; that is, a single image source is relayed to both eyes. Conceptual configurations are illustrated in FIG. 2 thereof, using a constant groove depth Fresnel, and disclosed as part of a claimed biocular viewing system in claims 14 through 23. However, constant groove depth Fresnels in combination with diffractive surfaces have proven to be problematic in that they can create unwanted chromatic artifacts. Microdisplay products are generally sealed between the display and eyepiece, therefore do not require two-sided Fresnel and diffractive surface protection from the user environment
FIG. 1 depicts a series of lens designs that illustrate the evolving advantages of various lens architectures. All are shown with display screen 1 on the left at the object plane imaged by various lens configurations through eye pupil 3 and eye lens 2 onto retina 4. Respective on-axis bundles 5a, 5b and 5c and peripheral ray bundles 6a, 6b and 6c are shown along with their corresponding on-axis spot diagrams 7a, 7b and 7c and peripheral spot diagrams 8a, 8b and 8c, and overall geometric distortion plots 9a, 9b and 9c. 
This series of example designs, while not intended to be exclusive, is illustrative of known lens architectures applied to viewing optics for a smartphone having a 5-inch diagonal display in a side-by-side binocular viewing mode. As such, one-half of the effective display screen 1 is approximately 28 mm×31 mm. All are designed with an effective focal length 10 of approximately 58 mm and an eye relief 9 of 25 mm to accommodate clearance for the user's prescription eyewear. The resultant diagonal field of view 13 is in excess of 70 degrees. An appropriate lens design will be capable of resolving a single pixel on the smartphone screen clearly over the entire field of view. A reference pixel 11 is shown on the spot size diagrams relative to the scale bar 12. This is illustrative of a 58 microns square smartphone screen pixel as perfectly imaged onto the retina, i.e., an effective image size of approximately 12 microns square.
FIG. 1A illustrates the performance of a prior art singlet glass lens 14a having spherical surfaces. Having very large geometric and chromatic aberrations, lens 14a is clearly not capable of resolving single pixels.
FIG. 1B illustrates performance of a prior art plastic lens 14b with dual aspheric surfaces. It offers significant improvements over simple glass lens 14a, but with significant geometric and chromatic aberrations it still lacks sufficient resolution to resolve peripheral rays 6b. Additionally, with a center thickness of 12.5 mm this would be an expensive lens to injection mold. Such designs are quite effective in a smaller format as used for lesser fields of view and/or smaller eye relief distances and are particularly effective in such less demanding applications when one surface includes diffractive features for chromatic correction. However, such designs are used in a number of products utilizing electronic micro displays, but are not well suited for large-field, large display products such as smartphones due to required lens thickness, weight, cost and inferior off-axis performance.
FIG. 1C illustrates the performance of a prior art lens doublet design including a continuous-profile Fresnel lens 15 on a planar substrate in conjunction with an aspheric corrector lens 16. Aside from its chromatic aberration, its performance is very good; that is, it capable of sharply resolving monochromatic pixels on a black background over the full field of view. However, this is insufficient, for example, for sharply resolving black text on a white background at the periphery of the field of view. Such text would rainbow with reduced contrast.